In recent years, the importance of lithium ion batteries, nickel hydride batteries and other secondary batteries as vehicle-mounted batteries and power sources for personal computers and mobile phones has increased. In particular, much is expected of lithium secondary batteries, which are lightweight and achieve high energy density, as batteries able to be advantageously used as vehicle-mounted high output power sources. A positive electrode including a lithium-transition metal composite oxide, a negative electrode including a carbonaceous material and a separator including a porous film between the positive electrode and negative electrode are provided in a typical constitution of this type of lithium secondary battery. Prior art relating to this type of separator is disclosed in Patent Literature 1 and Patent Literature 2.
Single layer separators consisting of porous polyethylene are widely used as the above-mentioned separator. However, single layer separators consisting of porous polyethylene have the advantage of exhibiting a shutdown function at an appropriate temperature (for example, approximately 130° C.), but have the drawback of causing battery performance to deteriorate due to oxidative degradation when exposed to the positive electrode charging potential. Furthermore, single layer separators consisting of porous polyethylene melt and deform at temperatures of 140° C. to 150° C., meaning that if the battery temperature increases even after shutdown, the separator may deform and cause an internal short circuit.
As a result, investigations have been made in recent years into the use of separators obtained by laminating polymer layers having different functions instead of single layer separators consisting of porous polyethylene. For example, Patent Literature 1 discloses a non-aqueous electrolyte secondary battery in which a separator composed of at least three layers, namely a heat-resistant porous polymer layer consisting of a polymer such as an aramid or a polyimide, a porous polyethylene layer and a porous polypropylene layer, a positive electrode is disposed so as to face the porous polypropylene layer, and a negative electrode is disposed so as to face the heat-resistant porous polymer layer. It is said that because the heat-resistant porous polymer layer in this non-aqueous electrolyte secondary battery has a high heat deformation temperature, the shape of the separator can be easily maintained even when thermal shrinkage and the like occurs. More prior art relating to this type of separator is disclosed in Patent Literature 2.